1. Field of the Invention
This invention relates to low bit-rate audio coding systems and more specifically to a method of improving sound quality of established low bit-rate audio coding systems without loss of decoder compatibility.
2. Description of the Related Art
Numbers of low bit-rate audio coding systems are currently in use in a wide range of consumer and professional audio playback products and services. For example, Dolby AC3 (Dolby digital) audio coding system is a world-wide standard for encoding stereo and 5.1 channel audio sound tracks for Laser Disc, NTSC coded DVD video, and ATV, using bit rates up to 640 kbit/s. MPEG I and MPEG II audio coding standards are widely used for stereo and multi-channel sound track encoding for PAL encoded DVD video, terrestrial digital radio broadcasting in Europe and Satellite broadcasting in the US, at bit rates up to 768 kbit/s. DTS (Digital Theater Systems) Coherent Acoustics audio coding system is frequently used for studio quality 5.1 channel audio sound tracks for Compact Disc, DVD video and Laser Disc and bit rates up to 1536 kbit/s.
A major problem with these systems is that their designs are inflexible in that they cannot be easily upgraded to accommodate higher PCM sampling frequencies, PCM word lengths or higher system bit rates. This will become an important issue in coming years as the music and film industry moves to drop the old compact disc digital audio format of 44.1 kHz sampling frequency and 16 bit word length, and adopt the new DVD audio PCM mastering format of 96 kHz sampling and 24 bits word length.
As a result, audio delivery using existing audio encoding systems such as AC-3, MPEG and DTS, must adapt to allow the benefits of this increased signal fidelity to pass to the consumer. Unfortunately a large installed base of audio decoder processing chips (DSPs) which implement these decoder functions already reside in the existing consumer base. These decoders cannot be easily upgraded to accommodate the increasing sampling rates, word size, or bit rates. Consequently, music and film content providers selling product through these mediums will be forced to continue to supply coded audio streams that are compliant with the old standards. This implies that in the future, delivery media such as DVD audio, ATV, satellite radio etc, may be forced to deliver multiple bit streams, each conforming to different standards. For example, one stream would be included in order to allow owners of existing playback systems to receive and play the standard audio tracks, while a second stream would also reside to allow owners of newer equipment to play audio tracks encoded using the 96 kHz/24 bit PCM format and take advantage of the inherently higher fidelity.
The problem with this method of delivery is that many of the playback mediums may not be able to afford the extra bandwidth, or channel capacity, necessary to send the additional audio streams. The bit rate of the additional bit streams (for example, those that support 96 kHz/24 bits) will be at least equal or greater than those that support the old format. Hence the bit rate will most likely double or more, in order to support two or more audio standards.